1. Field of the Invention
This invention is generally related to controlling build-up of polymerized TEOS in vacuum pump lines, valves, and other components downstream from a silicon dioxide deposition chamber, and more particularly to a trap containing a molecular species-selective flow impeding medium for adsorbing and retaining TEOS and water molecules long enough to consume substantially all the water molecules in hydrolysis of TEOS while allowing non-hydrolyzed TEOS, ethylene, and other gaseous byproducts in the effluent to pass through the medium and while trapping solid and liquid phase polymerized TEOS formed by the hydrolysis reactions for later removal and disposal.
2. Description of the Prior Art
Thin films of silicon dioxide (SiO.sub.2) are deposited on silicon wafers and other substrates for semiconductor devices with chemical vapor deposition (CVD) processes in which a source material comprising silicon and oxygen atomic species are reacted in a vacuum chamber to produce the silicon dioxide. As described in detail in U.S. Pat. No. 5,827,370, which is incorporated herein by reference, tetraethylorthosilicate or tetraethoxysilane gas (Si(OC.sub.2 H.sub.5).sub.4), also known as TEOS, is often used as the source material because of its excellent trench/via filling capability, low particle level, and resulting high filling quality for semiconductor devices. TEOS also has a high auto-ignition temperature, which makes it safer to use than silane gases.
In a typical CVD process, a vacuum pump is connected to a reaction chamber, the silicon wafer or other substrate is positioned in the chamber, and the chamber is evacuated and heated. A feed gas comprising the TEOS is then fed into the evacuated reaction chamber, where some of the TEOS is disassociated by pyrolysis into its atomic species, which recombine as SiO.sub.2 and other molecules, including water vapor (H.sub.2 O). SiO.sub.2 deposits on the substrate, while the remaining partially polymerized TEOS, H.sub.2 O, and other gaseous molecules (primarily ethylene (C.sub.2 H.sub.4) and any carrier or dilution gases, such as helium (H.sub.e2) or nitrogen (N.sub.2)) are drawn as effluent out of the reaction chamber by the vacuum pump. Oxygen (O.sub.2) or ozone (O.sub.3) are sometimes also used to reduce the reaction temperature in the furnace.
The vacuum pumps are connected to the reaction chambers by pipe sections called pump lines, and there are often one or more valves and other components in the pump lines between the reaction chambers and the vacuum pumps. The TEOS molecules are very unstable in the presence of the water vapor and hydrolyze easily into long polymer chains that form in the pump lines, clog valves and other components, and damage the vacuum pump.
The virtual wall provided by the gas boundary layer creating apparatus of U.S. Pat. No. 5,827,370, in which an elongated annular nozzle assembly provides a nitrogen (N.sub.2) boundary layer between effluent TEOS, water vapor, and other by-products of the reaction chamber, is effective to move those effluent byproducts away from the reaction chamber and further downstream in the pump line before the TEOS and water vapor polymerize on interior surfaces of the pump line. However, such polymerized materials (solidified or liquefied TEOS polymer) still have to be removed before they reach the vacuum pump and any valves or other components in the pump line that can be clogged and/or damaged by such materials. Various traps have been tried for such removal. The U.S. Pat. No. 5,827,370 indicates such a trap in a generic manner downstream from the virtual wall or boundary layer apparatus in that patent. However, such traps have not been very satisfactory, due to either clogging too fast, not effectively removing the TEOS and water vapor molecules, or both, and they are very difficult, if not impossible, to clean, yet expensive and time consuming to replace. Consequently, there is still far too much down time, labor, and expense incurred in dealing with the problems created by polymerized TEOS downstream from the reaction chamber.